By controlling the voltage across the plasma membrane, ion channels regulate a number of important cellular processes. Recent advances in electrophysiological techniques allow single ion channels to be studied under voltage-clamp in cell-free patches of native membrane. We are using this patch-clamp technique to investigate the molecular mechanisms through which ion channel activity can be modulated by the intracellular biochemical pathways that transduce extracellular signals into altered cellular responses. Initially, we have focused on the dihydropyridine-sensitive calcium channel that is activated by membrane depolarization in mammalian tumor cell lines. We have obtained the first direct evidence that these channels must be phosphorylated by the cyclic AMP-dependent protein kinase in order to respond to membrane depolarization. We have also demonstrated that, unlike other voltage-activated ion channels which inactivate during sustained depolarization, dihydropyridine- sensitive calcium channels only inactivate when channel activation leads to calcium ion entry and accumulation inside the cell. Our most recent experiments suggest that calcium promotes inactivation by stimulating an endogenous protein phosphatase to dephosphorylate the channel protein. These results provide a simple framework for understanding calcium channel regulation by extracellular signals, clinically important drugs, and environmental toxins.